Conventionally, in a working machine such as a combined harvester or a hydraulic shovel, there has been employed a technique of a so-called automatic low rotation control (an auto deceleration control) which automatically lowers an engine speed to a low idle rotating speed under a predetermined condition so as to achieve a low fuel consumption and a low noise (refer to patent documents 1 and 2). For example, in the patent document 1, there is disclosed such a matter that an engine speed is lowered to a low idle rotating speed if a grain threshing clutch connecting and disconnecting a power transmission to a grain threshing device is in an off state and a main shift lever for carrying out a shift operation of a traveling machine body becomes in a neutral state, under a state in which an idling selection switch is operated to be turned on, in a combined harvester. In the patent document 2, there is disclosed such a matter that an engine speed is lowered to a low idle rotating speed in the case that an operating lever operating a working portion (a bucket or the like) of a hydraulic shovel is not operated continuously for a predetermined time.
Further, in the working machine, there is a structure which employs a so-called forced low rotation control (a one touch deceleration control) which achieves a low fuel consumption and a low noise by automatically lowering the engine speed to the low idle rotating speed, on the basis of a push-down operation of a push button type switch which is arranged within a control portion (refer to patent document 3).
On the other hand, in conjunction with an application of emission regulations of high order relating to a diesel engine, it is going to be desired to mount an exhaust gas purifying device which purifies an air pollutant in an exhaust gas to a working machine on which the diesel engine is mounted. As the exhaust gas purifying device, for example, a diesel particulate filter (DPF) has been known (refer to patent documents 4 and 5). In the DPF, since a particulate matter in the exhaust gas is piled up in a soot filter due to an aged use, the soot filter is regenerated by burning and removing the particulate matter at a time of driving the diesel engine. As is known well, a soot filter regenerating action occurs in the case that a temperature of an exhaust gas is equal to or higher than a regenerable temperature (for example, about 300° C.).
If a state in which the exhaust gas temperature is less than the regenerable temperature carries over, a lot of particulate matters are piled up in the soot filter. As a result, the soot filter is clogged and an exhaust gas pressure rises up, thereby causing an engine trouble. As one example of a countermeasure for dissolving the problem, there has been known a forced regeneration control which enhances an output (a rotating speed or a load) of a diesel engine in the case that a depositing amount of the particulate matter reaches a predetermined amount, thereby forcibly raising a temperature of the exhaust gas conducted to the DPF so as to burn and remove the particulate matter collected in the DPF.